Fluorescent products and methods of preparation thereof



Feb. 15, 1949. B. s. ELLEFSON 2,461,726

- FLUORESCENT PRODUCTS AND METHODS OF PREPARATION THEREOF Filed Dec. 8,1942 2 sheets-sheet a 9 .IO .20 .30 .40 .so .so .10 .75

wf f fam INVENTOR Patented Feb. 15, 1949 FLUORESCENT PRODUCTS ANDMETHODS OF PREPARATION THEREOF. Bennett S. Ellefson; Emporium, Paa'ssignor to Sylvania Electric Products Inc., Emporium, Pa.,

a corporation of Massachusetts Application December 8, 1942, Serial No.468,262

This invention relates tofluorescent' materials which, underpredetermined excitation, emit light of a character causing the visualsensation of white. I

Heretofore, it has been proposed to produce the visual whiteeifect, byforming'the fluorescent material from a'mixture of phosphorswhich havecomplementary'hues. However, prior. to the present; invention, nocompletely reliable method of control has been proposed for; preparingthe phosphors with the desired'characteristics so that the final productpossesses the desired uniformity in producing a standard white visualeffect. After considerable study and experimentation, I have found thatin arriving attheproper mixing proportions, there area considerablenumber of factors which shouldv bev taken into account andaccuratelycontrolled if the desired uniformity of visual white is tobeattained. For example, in determining the optimum mixing ratio of twocomplementary fluorescent components, the following factors should becarefully considered.

1. Thef'absolute energy conversion of the materials in'the visiblerange. 1 I

2. Relativevisibility of the visibleradiation-s.

3. The effect of particle sizes on the effective visibility; v

Accordingly, itis a principal object of this invention to provide amethod of control and preparation of a mixture of phosphors to produce auniform standard white visual response, substantially independently ofcertain uncontro lable factors in the materials ,or in the commercialapplications thereof. 1

Another object is to, provide an improved methd of preparing twofluorescent materials which will yield, awhite fluorescent. hue whenmixed incertainfixed rangesof proportions. f

Another object is to provide a method of; preparation of fluorescentmaterials producing 'a white fluorescent response, whereby uniformity ohuefrom batch to batch is insured within the limits of huediscriminationof the human eye. Another object is to provide'a white fluorescentmaterial wherein the white responseis con trolled not only by the mixingratios of the two fluorescent components, but 'also' in accordance withthe ratios ofthe particle sizes of, the said components. Whileheretofore, it has beensu gestedto'mix complementary fluorescentmaterials to produce a so-called white hue, these prior suggestions havenot taught the necessary ratio factors of particle size whichmust'beconsid-f ered in arriving at, the'proper mixing ratio; I

havefound that'the mixing ratio is' greatly de- 8 Claims. (01.252-30115) pendent upon a satisfactory ratio of particle sizes inthe twocomponents. This, is particularly true in the case of fluorescentmaterials to be'excited by cathode-ray bombardment.

A further object isto provide a method ofpreparing the components for amixture of fluorescent materials so that over large batches theresulting fluorescent hue of the mixture'under electron bombardment willlie withinv the limits corresponding to the visual color ofla black bodycurve at temperatures between 6500 Kelvin and was Kelvin. j 1; 1

g A feature of the invention relatesto a method of preparing azinc'beryllium silicate'whose fluorescent hue coeificie'nts can beshifted so as to represent 'a'p'oint on a straight line on thechrcmaticity chart which passes through any two arbitrarilychosen'poi'nts of the black body curve located between thoserepresenting the temperatures 6400" K. and 2848" K.

Another feature relates to a method of preparation of a zinc sulfideWhose fluorescent hue coeflicients can be shifted so' as to represent apoint" on a straight line on the chromaticity chart which passes throughany two arbitrarily chosen points of the black body curve locatedbetween those representingthe temperatures 6500" K. and 2848 K. I

Another feature relates to a mixture ofa zinc beryllium silicate and azincsulfide which are prepared so that the fluorescent hue coefficientof the resultant mixture can be chosen at any point between thatcorresponding to a black body at a temperature between. 6500 K. and2848" K.

A further feature relates to a method of preparing a zinc berylliumsilicate-zinc sulfide white fluorescent mix whereby variations of theeffective luminous efficiency of the components in the mix result indeviations from a'standard white (e; g. that corresponding to a blackbody at 6500 K.) substantially along the curve ofvvisible radiation of ablack body at a temperature between 2843. K. and 6500" K. j.

'A further feature relates to a method of control in the preparation ofa white fluorescent mix" so that when the mix is appliedto devices suchas cathode-ray tubes, fluorescent lamps, etc., which are subjected tothe usual heat treatment or baking, the. green shift of the white. hueisreduced to a minimum.

A still further feature relates to a process of preparation of a whitefluorescent ,mix wherein the final product under electron excitationproduces a tangential white? as herein defined.

Other features and advantages not specifically enumerated will beapparent after a consideration of the following detailed descriptionsand the appended claims.

In the drawings, Figures 1, 2 and 3 are standard trichromatic colorcharts, useful in explaining the invention.

Development of white fluorescing materials has beenconsistently'stimulated during the development of television picturetubes of the cathoderay type. In these tubes the fluorescence of thematerial is excited by cathode rays impinging on the fluorescent screenat. a 'velocityficorresponding to a voltage difference of severalthousand volts. White fluorescingmaterials are also applied to the wallsofmercury vapori'discharge lamps in which the fluorescence is excitedmostly by the ultra-violet light generated in the mercy vapor discharge.

A preferred fluorescent material whichayields good white light emissionconsists of a mixture of manganese activated zinc beryllium silicatewith silver activated zinc sufide. Such a mixture has been described inmyprevious applicationfSerial No.'272,559, filed May 9, 1939, nowabandoned. The fluorescent hue of the silicate is yellowish, that of thesulfideis bluish-and the dominating colors 'of the components arecomplementary. As usual two colors are called here complementary "whenthey complement each other sofas'to' produce the sensation of whiteif'mixedin the proper intensity proportions.

As is taught in colorimetry, any given. color sensation can bereproduced by a superposition of three' different fixed lCOlOI stimuli'called primaries which may be chosen arbitrarily within limits. Oncethese three primaries have been chosen, any given colorfsensation can beexpressedinterms of three'coordinates or coefiicients which areproportional to the intensities of the three primaries.

These primaries have been standardized by the International Commissionon Illumination in such a manner that all colors can be expressed bypositive color. coordinates (trichromatic coeificientsiand theproportionality constant has beenso chosenfthatthe'sum of the threecoordinates of a" color "is" always'equal to unity. Accordingly,anycolor is completely determined by only two of the three coefiicients,say by :n and 11," because the.third"coefiicient'izl' is fixed by thecondition x+y+z==1. The I. C. I; primaries do not correspond'to realcolors existing in nature. Their 'choicewas determined by a number ofadvantages which could not be secured by any set of natural primaries.

These advantages are: First, any hue'is completely determined by onlytwo of the three coefficients; second, for a spectralcolor, one of thecoefiicientsis equal'to the visibility of that color, as" defined by thesensitivity of the human eye for the :different wave lengths of thevisible spectrum} third, all real colors can be expressed by positivecoordinates smaller than one; furthermore, the standard coordinates arelinear functions 'of other coordinates based on any set of primarieschosen from the range of real colors (within limits).

. As a result 'ofthislinear relation between the standard primaries andthe, spectrum colors,'the hue resulting fromthe mixture .of 'two givenadditive colors has trichromaticcoefiicients corresponding .to. a point.located on a straight line connecting the two points representing thecomponentcolorsj. Inthe standard color charts, :2: isthe 1.0. I. redcontent, u-is'thestandard visibility or the I. C. 1. green content ofany given "These three white standards are called,respective1y,jIl.luminant A,Illuminant B and Illuminant C. Illur'nina'ntA corresponds very nearly to the color of a black bodyat a temperatureof 2848 i'Ke1vin.iIlluminant"B corresponds to the color of a' black'bodyat 4800" Kelvin. Illuminant C to that of a black body at 6500".Illuminant C is very nearly-thecolor of average daylight and is,

in this sense, a reasonably absolute white standard.

Standards A and B are white in a psychologicalsense because they are ina region of certain artificial light sources which have been used forcenturies. "For further details of the standard chromaticitycoefficients and their calculations from colormeasurements made withstandard color filters, see Hardys Handbook-oi Colorimetry, press,Cambridge}; Massachusetts, 1936.

Usually a curve representing "the spectral colors in the (Ii-11]plane'is drawn into thestandard chromaticity' diagram, as shown inFig. 1. This diagram is'drawn toscale and the'curve 180--520--4l0represents'the curve of pure spectral colors whose wavelengths inlightwave units (mm are designated thereon. Theblue content of a givenpoint in this chart'is only given by implication; as the primary bluecontentof a hue is represented'by its z coordinate. The curverepresenting'the spectrum colors makes it possible to recognize theblue, green and red regions inthe x-'y plane. Chartsof this kind intowhich the curve for the spectrum colors are printed and-their wavelengths marked, can be bought on the market. They are practicallyindispensable for a convenientjgraphical representation ofcolor data.

As has been menti'oned'before, the I; C, Izrchromaticity coordinates areso chosen thatthe mixture of .any'two additive colors picked at randomproduce a sensation which corresponds to a point onthe straight line'connecting'the two points representing the arbitrary. picked colors inthe ar.y plane of the'chromaticity'chart.

Referring tdFig. 1, let one of the components of the fluorescentmaterial be soichosenwith a color on'the blue side'of point C, sayatC online CB, the other with acolor on. the yellow side of point B,sayfB'..on,line BC. The line CBintersects the spectral curve at the redend iatapproximately. 577.7. mmp. .This .line also intersectsthespectral curve at'the blue end at approximatelyv 477.8 min I Thelinewhich. istangent at point Aintersectingthe black body curve at. A.and a point. immediately adjacent thereto lying between..A...andB-intersects the .spectral curve at approximately586 mm and at theblueend. at. approximately. .490 mm. The resulting sensationof two stimuli.having the coordinates C5. and B will belocated onfltheline connectingthe. points'B. .andIC'. The exact-.location of the resulting sensation:on line' B. and. C'dependson themixing ratio of the twoicomponents. Togive a concreteexampleassume that 1150-50 mixture produces a sensationcorresponding .to a point exactly. half way. between zpoints- B, and

' menace:

(l-'th'en'. the-addition of a little toward the point C; Inversely; asmall: ex-- cess of 33'' will shift the'coordinatesiof "theresultingsensation towardlpoint' B. Any variation of thev original ratio will,therefore, shift the coordinates, of: the resulting sensation from one:standard white, say C, to another standard white; say 13, substantiallyalong thecurve of blackebqdy radiationq: l

Ihave found that the processing oftwo-components of afiuorescent'mixturecan becarried out;in such a vway'that theirrespective fluorescent hues are; represented by two i points locatedi'ona linepassingthrough the I. C. I. standard-whitepoints B andC. Thisobjectivel can beattained in accordance with thejfOHOWiHgdiSCIOSUI'QS.

The coordinates of-the silicate component can be shifted between "theyellowish and greenish regions by varying the ratio of zinc toberyllium, and-,by-yarying the firing time and-the activatorconcentrations; The.;fluorescent hue of the sul-.

fide component, on the other-hand, may beshifted back and forth betweenthe v,lcluish' andigreen-q ish-region by. theproperchoice of activatorsand their concentration and of firing time and temperature. ,gThe pathsof the chromaticity points of the two components obtained by these-processing variationsare substantiallyv perpendiculart to thei; i eBClin .the'chromaticity chart of:

. herefore, by properprocessing control the points of intersection ofthese pathswith' milled for three hours and fired at 9002C. for

minutes. a I

, It'isext-remely important that the ingredients are very pure becausethe energyconversion of the sulfide material under excitation is verysensitive'to impurities, in particular against iron and nickel. Thesensitivity of the silicate is in this respe ct;not. qute as high asthat. of the sulfide in which a few 'parts in a million will alreadyreduce the luminescent efiiciency very: markedly. The sulfide inaddition is very'sensitive to grinding,-i e.," to pressure and shock.Milling above tWo hours produces a marked loss in ef ficiency. "Thesilicate is only moderately sensi tive against grinding but the grindingtime must be kept below hours in the average ball mill in order to avoida decrease in 'luminescent efiiciency. 1-2 j II. A correspondingsilicate component is obtained as follows. ZnO, BeO and SiOz orcompounds which decompose'into them. on subsequent heating are wellmixed insuch proportions as to yield in the final product 40 mol percentZnO, 23 molpercent BeO, ,37 mol percent $102. To these compounds isadded MnFz in a'quantity such" that in the final luminescent materialap-- proximately-1.61 manganese byweight willbe present These rawmaterialsare wellinixed smallquantity 'of com-". ponent .Cwill-"shiftrthe-hue of the mixture a lfidefroni purified. zinc sulfatetowhich fied melt is crushed and then ground bymilling nents.

and 'themmelted satastemperature fibetween' 1400 and 1500 C.Aftercooling; thepartlyrdevitri in "a porcelain .ball milliusing acetoneas a milling vehicle. After thezmate'rial'has been reduced to." a'fineness f of thirty microns or less, the dried material is reheated at1000 C. "for a period of minutes The time-depends somewhat on the size'of the crucible. :After this low: temperature:

reheating, thematerial is again'milled in-a conventional ballmillrfor afew hours to break up the;%sintered; particles resulting from. thesecond heat treatment, resulting in a wide range of particle sizestfrom100.microns down to 01 micron.

While vthetrichromatic 'coeificients representing-"each-component can bekept with sufficient;

accuracy within therlimits desired for successful location on the1ine:B,C, it is impossible to secure uniform energy conversion frombatch, to batch:

(A possible color shift of the. sulfide, component due tobakingofthematerial after application.

to a glass .wall or other support, will be discussed later) Thevariation of luminous efficiency is substantially; due to'two factors.The, twocomi l ponents must be mixed for proper distribution.

of-.the materials "before application to the screen.

The mixing operation itself 'will infiuenceithe luminousflefficiency ofthe-sulfidev due to sensiavoid The sulfide ponentare basicallydifferentmaterials, and their surfaceiproperties, su-chas encountered in col-.--"loids; are for this reason COIlSidGIabIy difierentn Surface. properties.that determine dispersion conditions in a liquid are, in addition, verysusceptible to extremely small variations. inpro-;

cessing conditions; These variations in surface.

properties, and the corresponding variation in dispersability aresignificant in screen application methods'in which preferential settlingfrom a suspension containing two components is a.

factor in changingthe ratio of components. :ac,'-. tually reaching thesurface to be screened. This variation in actual ratio would also act ina manner similar to diminution or reenforcement -',ne'cessarily beaffected in the same manner. Another factor which enters intoreproduceability of color .isthe effective ratio of compo- This is dueto variations in particle size of the milled silicate component evenunder reafi'sonably well controlled conditions of milling. A

- given weight ratio of components would not consistently producematched surface ratios exposed f to the elec'tron'beam excitation underthese conditions.

The chart which carries the standard trichromatic diagram also carriesthe curve of black body radiation (dot-dash curve). Three lumicurvatureof the black body curve between points C and B of the I. I. color chartis very slight, and a shift ofapoint from B to 0 along. the straightline BC is for all practical purposes idenf tical-with a shift fromB toC along the black; bodycurvef The white fluorescence resulting componentand the silicate 'em-.

incomes:

from the mixture ctr-components .according to the invention ,istherefore. identified herein as v bymixinglblue sulfides and: yellowsilicates, which producefluorescent hues located on other straightlines-passing through two neighbouring points ofthe'i black body curvewithin the range of the standard illuminants A, Band C. An example isthe location of the chromatic representative points :of the componentsof the mixture on line AB '(Fig; '2) which maybe considered-asat'olerance line for a tangent drawn through a: point T. of. theblackbody curve as shown inFig. 3. The tolerance is justified by the factthat the human eye doesv not discriminate more than about 160diiferentcolor shades, and a slight'deviation of a hueabove or below theblackwbody curve is, therefore,- not noticeable it limited to areasonable. value. V

.In contrast to these tangential whites, "intersecting" whites may beproduced by mixing componentsof a fluorescent material which havedominant spectrum colors located on any of the straight linescorresponding to selected pairs of complementary colors, twelve of whichhave been defined by Sin'den in 1923. Review of Scientific Instruments,volume 11, pages 1123-1153, published by 'theAmerican Institute ofPhysics, New York, New York. In particular, Sindens pair N052, whichgives theoretically the maximum relative'energy efliciency, is such anintersecting-white. The corresponding straight line passing through lineB has been drawn into Figs. 1, 2 and 3, as well as the linecorresponding to Sindens complementary pair No; 12, which-yields also anintersecting" white.

I'have found that the cooperatingcomponent fluorescent materials withhues located on the linemarked Sinden -2- on the chromaticity chartmaybe obtained by mixing sulfide VII-with 2 ,are' more critical; and resultid mor notice able deviations from desirablemwhite hues, than mixtureIpluslI, which hasbeen discussed, above BC white, or :the mixtures IIIplus IV, or of I with either IVo, V, or VI of the processing table givenhereinbelow. As a result, uncontrolled variations' of :the effective:luminous efficiency of the components of a white fiuorescentmixtureresult substantially in a deviation from any .of the white pointsbetween A and C originally chosen :as standard to another point'of theblack body cur-ve also located between-A and. C.

According to the-status of the art, it'lis assumed. that the choice 'of'thecomponentmaterials for a white mixture is notcritical as long as theyare chosen in pairs so as to represent complementarycdlors in the rangebetweenSinden 1 and Sinden 12. -What I believe is new in-myfin'dingx'above the prior art, may be summarized as follows.

. l. Itis-not essential to select: the pairs of-component materials so"that their respective hues arecomplimentary in the sense definedbyjSinden, ire. withrespect to point B.

1 2. The range of complementary pairs is critical, and mustbechosen soas to-conformwith the rules of a tangential white. as d'efined herein.

The following table gives an outline of some of the processing methodswhich-yield components for various white mixtures according to theinvention.

The numerical influence of the increase of BeO, activator percentage andd'evitriflcationperiod of the hue shifting of the-silicatecomponenttoward longer wave lengths is clearly--indicated in the table.

Similarly, the table givesanumerical guide for the shifting of the hueof the sulfidecomponent toward shorter wave lengthswith increase of thesilver activator percentage. A shiftin of P the hue of the sulfidecomponent fluorescence toward longer wave lengths as indicated in thetable, can be obtained by adding a copperactivator, which,'as iswellknown, will also increase the period of phosphorescence of thesulfide.

Processing table III in Sulfide Fired at 900 C. for 25 minutes, particlesize 0.1 to

. (0.003%- Ag plus) 1.0 micron. 0.000595 Cu) IV ZnBe Silicate ZnO 43 molper cent Melted at 1400" to 1500 C. 'Devitrifled at .1000",

* (0.50% Mn) BeO 20 0,, l-4 hours.

SiO: 37

IVa ZnBeSilicate Z120 40 mol per cent Melted at l400 to 1500" C.Devitrifled at l000 (2% Mn) Bot) 23 0., for l-4 hours.

.SiOr37 V ZhBeSilicate 'ZnO 37 mol per cent Solid phase reactionl280?Ctlor 2-0 hours, particle (1% Mn) BeO 27 size selected 6 microns to 0.1micron.

' SiOa 36 VI ZnBe Silicate Z110 37 mol per cent Particle size, 16microns. to 0.1 micron,

( Mn) BeO 27 sic; as

' VII Zn Sulfide I Fired at 900 0'. for 25 minutes, particle size 0.1 to

. %.A 1.0 micron.

VIII znB Silicat ZnO 49 mol per cent Solid phase reaction 1280 0. for2-6 hours, particle (1% Mn) vBeO l5 -,sizes 3 microns to 0.1 micron.

S10: 36 IX ZnBe Silicate Z110 49 mol per cent Solid phase reaction .1280C; for Z-fihours, particle (1% Mn) sizes 6 microns to 0.1 micron.

either of the silicates VIII-or IX processed as indicated in theprocessing tables given hereinbelow. 'I have also found,, that. mixtures.made

from; cooperatingcomponents followingfiinden s:

To produce tangential white the, preferred mixing ratio of sulfideIv(column 5 supra with silicate II (columns 5 and 6, supra) or-oneof thesilicatesIVd to VI isflbetween thelimits 1:2 -.and

spec-n6 :3; roi'sumunrefid l' ifie preferred mixing ratio-r3133.

The preferred mixing ratio sulfide V1:

charge lampsa'nd such electronic devices, it is necessary to bake thetubes-during the exhausting'process-inorder to remove adsorbed orabsorbed gases which .might later. be given off and affect the life'of'the tubes." Itis desirable to .a-bake 'atthe: maximum temperaturewhich is determined either by the material-of the -.en velope or by thecomponent parts of the tube in order"toaccelerate the removal of gas. Ina device in which fluorescent zinc sulfide is one of the componentparts, the'zincsulfide becomes 'th'e limiting factor. for the :bakingtemperature. .At a temperature of 350. C.'---3'75 C., the

fluorescent zinc sulfide undergoes a gradual but yellow fluorescentcolor" complementary to the original blue of the sulfide'gisi therefore,a slight" shift from the original white in the 'greenish direction. Whenthe original spectrum colors of the two components are chosen so thatthey fall approximately on a 'line going through points B and thiseffect is very much reduced.

This is partly due to the fact that the shift of the blue sulfidecomponent towards the greenish side as a resultof baking during exhaustis less pronounced if the sulfide has been so processed that itsdominant pro-exhaust hue does not correspond to the very short (blue)frequencies of the spectrum colors characteristic of the sulfidecomponent needed for the subsequent maximum white efficiency of themixture of silicate with sulfide. As far as I am aware, this feature hasnot been disclosed heretofore though it is of great practical importancein the manufacture of commercial devices such as cathode-ray tubes,fluorescent lamps, etc.

While in the foregoing disclosure reference has been made to mixtures offluorescent materials which are composed of sulfides and silicates whosevisual hues mix in an additive relation, it is understood that the scopeof the invention includes also the application of the tangential whiteprinciple employing compo- "6f substantially -ecmp1ementary""fluorescent "colors, *the respective trichromatic coeflicient points ofwhich on'said'chart define a straight line which is substantiallytangent to the said black body curve at'any' point between 2848 K.

and 6500 the limits of the complementary I dominant wavelengths atf'thered end of. I the spectrum being between 577.? mm and 586 mm ---'and thelimits-of the saidcomplementary dominant wave-lengths at 'theblue end ofthe spectrum being between 477 8 mm and490 mmc and mixing said twophosphors to produce a resultant fluorescent color having trichromaticcoeffi- 2. The method of producin f fluorescent white mixture withsubstantially uniform reproduce 'ability of fluorescent whitecolor inaccordance with a trichromatic coeflicient chart such asillustrated-inthe accompanying drawing and onw'hich is plotted the curve of black bodyradiation'jbetween 284=8 Kfand 6500-K., which includes the I steps'ofselecting two fluorescent phosphorsof substantially complementaryfluorescent 'colors, the respective trichromatic coefficient points" ofwhichon said chart define a-"selecte'd: straight line which is'tangential to Said bl ackbo dy curve at the point whose trichromaticcoefficient determines the desired color to be produced, the limits ofthe spectral colors of said materials at the blue end of the spectrumbeing determined by the intersection with the spectral color curve onsaid chart of a first straight line tangent to the black body curve atpoint 2848 K. and by another straight line which intersects saidspectral color curve and which is tangent to said black body curve atthe point 6500 K., and mixing the phosphors in a ratio determined by thedistance of their respective trichromatic coefficient points from saidpoint of tangency on said selected straight line.

3. The method of producing a fluorescent white mixture withsubstantially uniform reproduceability of fluorescent white color inaccordance with nents in which the radiation produced by one 7 of thesecomponents is partly absorbed by the other component, or where eachcomponent partly absorbs the radiation emitted by the other. Also theinvention includes within its scopethe use of mixes of the tangentialwhite character in which the radiation emitted by one of the componentsstimulates visible fluorescence or phosphorescence in the othercomponent.

Various changes and modifications may be made herein without departingfrom the spirit and scope of the invention.

This application is a continuation-in-part of application Serial No.272,559, filed May 9, 1939 (now abandoned).

What I claim is:

1. The method of producing a fluorescent white mixture withsubstantially uniform reproduceability of fluorescent white color inaccordance with a trichromatic coefficient chart of the type illustratedin the accompanying drawing on which is plotted the curve of black bodyradiation between 2848" K. and 6500 K which comprises selecting twofluorescent phosphors a-trichromatic coefficient chart such asillustrated in the accompanying drawing and on which is plotted thecurve of black body radiation between 2848 K. and 6500 K., whichincludes the steps of selecting a bluish fluorescent phosphor having aspectral color at the blue end of the spectrum whose limits are definedby the intersection of two straight lines with the spectral color curveon said chart, one of said lines being tangential to the black bodycurve at the point 2848 K. and the other line being tangential to theblack body curve at the point 6500 K.; selecting a yellowish fluorescentphosphor having a spectral color whose limits are defined by twostraight lines, one of which is tangential to the black body curve at6500 K, and the other of which is tangent to the black body curve at2848 K.; selecting any desired straight line which istangential to saidq black body curve between said points 2848 K. and 6500 K., and whosepoints of intersection with said'spectral color curve are locatedbetween the aforementioned limits, selecting a first point on saidselected line to one side of its point of tangency with the black bodycurve, selecting a second point on said selected line on the oppositeside of said tangency point, and mixing said two phosphor powders inaccordance with the distance between each of said first and secondpoints with respect to said point of tangency to produce a resultantmixture whose resultant fluorescent color has trichromatic coefficientscorresponding to said point of tangency.

4. The methodaccording to claim :1 vin which manganese-activated zinc.beryllium silicate and silver-activated zinc sulfide are selected asthephosphors, the zinc berylliumusilicate having an excess ofsilica, themolar percent ZnO to BeO being between 1.37 to 1 and 3.26 to 1, saidphosphors having a particle .size of between 15 and 0.1 microns, thesulfide having been fired atapproximately 900"v C. and the silicatehavingbeen melted at a temperature between 1400 C. and 1500 C., anddevitrifled approximately 1000 0., and the said powders being mixed in aratio between and including 1:2 and 1:3.

5. The method according to claim 4 in which the-silicate powder ischosen with a 37 mol. ZnO, 27 mol. BeO, and 36 mol. SiOz.

6. The method according to claim 4 in which the zinc beryllium silicateis prepared so as to contain 43 mol. ZnO, 20 mol.'% BeO, and 37 mol.S102.

'7. The method according to claim 4 in which the zinc beryllium silicateisprepared so as to contain49 mol.. ZnO, 15 mol.. %.-Be0, and36 mol.SiO2..

8. The methodaccording to claim'l in which one of the phosphorsisprepared as a silver-acti- 12 vated sulfide containing,0.0005% silver byheating it at approximately 900 {or about 25 minutes, the other phosphorbeing prepared as almanganese-activated beryllium zinc silicatecontaining 6 37 mol. ZnO, 27 mol. .BeO, and 36 mol.

S102, which is heated from 2 W6 hours .at approximately 1200 C.; and thetwo phosphors are mixed in a ratio of from 1:2 to 1:.3.

BENNETT S. ELLEFSON.

REFERENCES. CITED The following referencesare 'of record in 'the file ofthis patent: I

15 I UNITED STATES PATENTS Number Name Date Re. 12,812 Hammer June16,1908

907,598 1 Hewitt Dec. 22, 1908 2,118,091 Leverenz May 24, 1938 202,171,145 Leverenz Aug. 29,: 1939 2,219,929 Kaufmann Oct.: 29,' 19402,243,097 Henderson .May-2'Z, 1941 FOREIGN PATENTS 25 Number. CountryDate:

480,356 Great Britain Feb. 22,- 1938

